1. Field of the Invention
The present invention relates to a heat-sensitive flow rate sensor for use in measuring flow rate and flow velocity of a fluid and, more particularly, to a heat-sensitive flow rate sensor of the type which measures flow velocity and flow rate of a fluid based on the rate at which heat is carried away from a probe by the fluid which flows in contact with the probe.
2. Description of the Related Art
FIG. 4 schematically shows the construction of a known heat-sensitive flow rate sensor of the type which is disclosed Japanese Utility Model Laid-Open No. 61-108930. A sensor tube 2, which forms a part of the fluid passage, is provided at a predetermined position in a housing 1 which defines a principal passage for the fluid. A measurement resistor unit 3 including a heat-sensitive resistor 6 (see FIGS. 5A, 5B), as well as a fluid temperature sensor 4, is disposed at predetermined location in the sensor tube 2. The measuring resistor unit 3 and the fluid temperature sensor 4, together with resistors R1 and R2, form a bridge circuit. The junctions h and c of the bridge circuit are connected to a differential amplifier 101. The output of the differential amplifier 101 is connected to the base of a transistor 102. The transistor 102 is connected at its emitter to a junction a of the bridge circuit and at its collector to a power supply 103.
FIGS. 5A and 5B are a front elevational view and a side elevational sectional view of an example of the measuring resistor unit 3 of the heat-sensitive flow rate sensor. Referring to these Figures, the measuring resistor unit 3 has a substrate 5 made of an insulating material such as alumina on which is formed a heat-sensitive resistor 6 in the form of a film. The heat-sensitive resistor 6 is made of a material which varies its resistivity according to temperature, and more specifically, to a material having a positive temperature coefficient. A patterning line wiring 7 is laid on the heat-sensitive resistor 6 so as to provide a path of electrical current. Lead lines 8 are connected to an end of the resistor 6. A protective coat 9 is formed on the heat-sensitive resistor 6 so as to protect the latter. The measuring resistor unit 3 is supported in the detecting tube 2 by a support portion 10. The operation of this known heat-sensitive flow rate sensor is as follows. When flow of a fluid at a constant flow rate exists in the housing 1, the bridge circuit is balanced in such a manner that the mean temperature of the heat-sensitive resistor 6 of the measuring resistor unit is maintained at a level which is higher than the fluid temperature by a predetermined value, by the control of the electrical current supply to the bridge circuit. The control of the electrical current supply is performed by a control circuit constituted by the differential amplifier 101 and the transistor 102. When the flow rate of the fluid is changed, rates of heat conduction to the surfaces of the heat-sensitive resistor 6 and the supporting substrate 5 changed. This change varies the temperature of the measuring resistor unit 3, causing a corresponding change in the resistivity of the measuring resistor unit 3, so that an imbalance is caused in the bridge circuit. The control circuit then operates to increase the electrical current supplied to the bridge circuit. Consequently, the heat-sensitive resistor 6 is heated so that the mean temperature of the resistor 6 is elevated to the level exhibited before the change in the fluid flow rate, whereby the bridge circuit is balanced again. The level of the electrical current supplied to the measuring resistor unit is used to measure of the flow rate of the fluid. The fluid temperature sensor 4, which is held by another supporting substrate and which is made of a resistor having temperature-dependency of resistivity, provides compensation for change in the output which otherwise is caused by a change in the fluid temperature. In the known heat-sensitive flow rate sensor having the described construction, the heat generated by the heat-sensitive resistor 6 is not only carried away by the fluid, but part of the heat is transmitted to the supporting portion of the measuring resistor unit 3, while other parts of the heat are changed into radiation energy and are dissipated as electromagnetic waves. Consequently, such a temperature distribution is developed over the measuring resistor unit 3 that the temperature is high in the region near the free end and low at the supported or base end of the measuring resistor unit 3, as shown in FIG. 6. Such a temperature difference between the free end and the base end of the measuring resistor unit 3 is further enhanced when the heat-sensitive resistor 6 is made of a material having a positive temperature coefficient of resistance, with a series type patterning wiring 7 in which the major pattern of the wiring 7 extends perpendicularly to the longitudinal axis of the measuring resistor unit 3 as shown in FIGS. 5A and 5B. This is because in such a case the resistivity of the heat-sensitive resistor is locally elevated at the high-temperature region and locally lowered at the low temperature region, with the result that the high-temperature region is further heated as compared with the low-temperature region, thus enhancing the temperature differential between the free end and the base end of the measuring resistor unit 3. Since, the mean temperature of the heat-sensitive resistor 6 is controlled to a constant level by the control circuit. Thus, a large temperature gradient is formed over the heat-sensitive resistor 6 because of locally developing a high temperature differentials. The rate of heat radiation increases in proportion to the fourth power of the absolute temperature. In general, the temperature gradient along the heat-sensitive resistor 6 is greater when the flow rate of the measured fluid is smaller, so that the temperature dependency of the heat radiation rate increases when the flow rate is small. When the rate of heat radiation increases, the proportion of the radiated heat to the total heat generated by the heat-sensitive resistor 6 increases correspondingly, so that the temperature coefficient of the heat transfer coefficient, as well as the flow-rate dependency of the temperature coefficient of the mean heat transfer coefficient, is increased. This results in error in the measurement, particularly when the temperature and flow rate of the fluid are changed. Thus, the known heat-sensitive flow rate sensor can not accurately respond to changes in the temperature and flow rate of the fluid to be measured.